The invention relates to a magnetic shield for a cathode ray tube.
All monitors or televisions which use a cathode ray tube (CRT) have an auto-degaussing feature which is activated every time the CRT is switched on. Degaussing of the CRT is necessary when, for example, the CRT moves locations to compensate for variations in the Earth""s magnetic field between different geographical locations.
The CRT includes a color identification device commonly referred to as the aperture grill assembly and a magnetic shield which is fixed to the Aperture grill assembly, normally by spot welding of a skirt, which extends around the periphery of the magnetic shield, to the edges of the aperture grill assembly. However, when a CRT is degaussed, there is a tendency for the edge of the skirt to vibrate and strike the aperture grill assembly. This causes a ringing sound which is audible and therefore, undesirable. In addition, there is also a tendency for the magnetic shield to ring if it is used in a television, as certain sound frequencies may also cause vibration of the skirt and therefore, also cause ringing.
The vibration could be reduced by reducing the degaussing force or by reducing the size of the skirt. However, if the degaussing force is reduced, it is not possible to degauss the CRT properly. In addition, if the end of the skirt around the magnetic shield is reduced, this reduces the amount of magnetic shielding.
This is also undesirable as if the magnetic shield is reduced, the CRT is more vulnerable to the effects of terrestrial magnetic fields. Therefore, if the CRT is not properly-shielded, the electron beam may be influenced by the terrestrial field and may cause the electron beam to strike the wrong section of the screen and distort the image. For example, the electron beam may strike a blue phosphorous region instead of a red phosphorous region, and so cause color distortion.
In accordance with the present invention, a magnetic shield for a cathode ray tube comprises a body portion and a skirt section around the body portion, the skirt section comprising a skirt portion having a free edge and being connected to the body portion at its opposite edge, wherein the length of the free edge is less than the length of the opposite edge.
Preferably, the edges of the skirt portion between the opposite edge and the free edge are at an oblique angle to the opposite edge and the free edge such that the skirt portion tapers from the opposite edge to the free edge.
Alternatively, the edges between the opposite edge and the free edge may comprise a number of stepped edges such that the free edge has a length less than the length of the opposite edge.
In a further example of the invention, the edges extending from the opposite edge to the free edge may be curved or have any other suitable shape.
In accordance with a second aspect of the invention, a cathode ray tube assembly comprises aperture grill assembly coupled to a magnetic shield structure, the magnetic shield structure being in accordance with the first aspect of the present invention.
Preferably, the separation of the grill assembly and the free edge is greater than             FL      3              3      ⁢      EI        ,
where F is a force applied to the skirt section, L is the maximum distance from the coupling of the skirt portion to the aperture grill assembly to the free edge, E is the Young""s modulus of the skirt portion and I is the moment of inertia of the skirt portion. For example, the force may be generated by degaussing, sound or any other means.
Typically, the length of the free edge is preferably less than 20 mm, and most preferably less than 15 mm.